Electron beam tubes



4, 1956 w. J. DODDS ELECTRON BEAM TUBES 2 Sheets-Sheet 1 Filed March 15, 1951 Wellegey "E2318 WWW 4 2/ Ill 7 -A -J I 1956 w. J. nouns 2,773,213

ELECTRON BEAM TUBES Filed March 15, 1951 2 Sheets-Sheet 2 INVENTOR WllcgleyJflodds TTORNEY United States Patent ELECTRON BEAM TUBES Wellesley .l'. Dodds, Allentown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 13, 1951, Serial No. 215,236

17 Claims. (CL 315---3.6)

This invention relates to improvements in electron beam tubes, and particularly to beam tubes of the traveling wave type.

In a conventional traveling wave tube an elongated section of transmission line with suitable inputand output terminations is mounted within an evacuated envelope. The terminations are sealed through the envelope or otherwise adapted to be coupled to external transmission lines. The transmission line section is designed as a delay line along which electromagnetic waves are transmitted at a fraction of the velocity of light. The form of delay line usually used in traveling wave tubes is a conductive helix, such as a helical metal coil. An electron bearn is projected by suitable means along, and preferably coaxial with, the helix at a beam velocity substantially equal to the axial wave velocity along the helix. In operation of the tube as an amplifier, a signal wave traveling along the helix creates electromagnetic fields therealong which interact with the electrons in the beam to produce electron velocity modulation and consequent electron bunching. As the wave and beam travel synchronously along the helix the phenomenon reverses and the bunched beam induces fields and currents along the helix. The amplitude of the wave increases along the helix, ecause the electron beam gives up more energy to the helix than it abstracts therefrom, thus producing an amplified signal at the output end of the tube.

The axial velocity of a wave along a simple helical coil designed for very high frequencies and having practical diameter and pitch is determined largely by the diameter and pitch of the helix and the velocity of light, and is substantially independent of the frequency of the wave transmitted. Therefore, the conventional traveling wave tube described is inherently a very broad band amplifier, the amplifying bandwidth of the tube being very often larger than the bandwidth of the input and output structures. The most favorable types of helices from the standpoint of high gain at low voltages and short overall length have so wide a frequency range of operation that it is difficult to match the helix to the output circuit over the helix bandwidth. If this match is not made a very low reflecting one over the required range, although the desired signal frequency or frequencies may see a reflectionless path into the output circuit, disturbances at other frequencies outside the band-pass of the output circuit will be reflected back and forth between the input and output circuits. These disturbances, which may be unwanted signals or shot noise in the beam, can be amplified to as great a degree as the desired signal and cause regenerative oscillations which distort and amplitude modulate the intelligencecarrying signal for which the input and output circuits are designed.

The principal object of the present invention is, therefore, to provide improved means associated with the helix for limiting the amplification bandwidth of a helix type traveling wave tube without producing undesirable oscillations.

2,773,213 Patented Dec. 4, 1956 A feature of the invention is the provision of such bandwidth limiting means in the form of an external filter helix or other transmission line or lines inductively coupled to the active helix of the tube;

In general, the bandwidth limiting means employed may be any external filter structure or structures coupled to the tube helix which will produce periodically-recurring discontinuities in characteristic impedance along the helix, and thus produce a helix-filter combination which is dispersive because the wave velocity along the helix is not independent of frequency. The tube is in effect pretuned during manufacture to a. particular frequency, or band of frequencies, at a given beam voltage.

The term dispersive as used herein is intended to describe a helix or other transmission line, or a combination thereof, having substantial dispersive properties, as distinguished from a substantially non-dispersive simple helix of uniform diameter and pitch.

The external filter coupled to the active helix may be a dispersive helix such as the various forms of periodically-loaded active helices disclosed in the copending application of W. l. Dodds and R. W. Peter, Serial No. 169,674, filed June 22, 1950, and assigned to the same assignee as the instant application. The present invention provides different means for accomplishing the same result as that of said copending application, that is, of limiting the amplifying bandwidth of the helix.

Alternatively, the external filter may be a non-dispersive helix, like the inner, active helix, provided the length of each helix is substantially an integral number of half wavelengths at the desired operating frequency and the two helices are designed to have substantially the same axial wave or phase velocity.

The external filter may also take the form of a branching filter composed of two or more transmission lines coupled to the active helix at equally-spaced points therealong by coupling loops. The branching filter is designed to extract from the active helix circuit those frequencies closely adjacent to the desired operating frequency band without affecting the operating frequencies.

Preferably, the input and output of the tube will be coupled to the inner, active helix, as in conventional traveling wave tubes. However, it may be desirable to couple signal energy into and out of the outer helix. The input may also be to the inner helix with the output taken from the outer helix, or vice-versa. In such cases, the outer helix must have substantially the same axial phase velocity as the inner helix, and the free helix ends should be suitably terminated.

In a copending application of R. W. Peter, Serial No. 187,946, filed October 2, 1950, and assigned to the same assignee as the instant application, a cartridge type traveling wave tube is disclosed wherein the active helix is disposed outside the tube envelope and a non-interacting, direct-current shield, which may be another helix, substantially transparent to high frequency fields, is disposed inside the envelope, between the electron beam and the external interacting helix, to stabilize the envelope wall potential. The input and output means of that tube are coupled to the external active helix. However, neither the external helix nor the shield helix nor the combination thereof is dispersive, there being no attempt made to limit the bandwidth of the active helix as in the present invention.

The objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention taken in connection with the annexed drawing, in which:

Fig. l is a longitudinal sectional view of a traveling wave tube embodying one form of the invention;

Fig. 2 is a transverse sectional view taken on line 22 of Fig. 1;

Fig. 3 is a graph used to explain the operation of the tube of Fig. 1;'

Fig. 4 is a detail view of another embodiment of the invention;

Fig. 5 is a longitudinal sectional view of still another embodiment;

Figs. 6-10, inclusive, are diagrams and graphs used to explain the operation of the tube of Fig. 5; and

Fig. 11 is a detail view illustrating an alternative method of coupling energy into and out of the tube made according to the invention.

Referring to the drawing, Figs. 1 and 2 show one form of traveling wave tube embodying the invention. The tube comprises a conventional elongated metal helix 1 of desired diameter and pitch disposed within an elongated tubular glass envelope 3, close to the inner surface thereof and supported thereby. Any suitable means may be used to couple an input signal wave to the helix 1. A waveguide 5 is shown coupled by a ring 6 to an axial extension 1' of the helix and connected to a tubular metal shield 7. The waveguide 5 is sealed at 8. A cup-shaped gun-receiving envelope portion 9 of the envelope 3 contains a gun structure 11 comprising a cathode and a series of beam-forming electrodes for projecting an electron beam through and in energy-coupling relation with the helix 1. An output waveguide 13 is coupled by a ring 15 to an axial extension 1 of the other end of the helix 1 and connected to the shield 7. The waveguide 15 is sealed at 17. A collector 19 is mounted within the output end of the envelope 3. Suitable operating potentials are applied to the various electrodes, by a battery 21, for example. The axial extensions 1 and 1" are coupled to the transverse electric components of the electromagnetic fields within the waveguides 5 and 13.

In accordance with the invention, the active helix 1 is inductively coupled, along the major portion of its length, with an elongated metal filter helix 23 which is disposed on the outside of the envelope 3. The purpose of the filter helix is to produce a dispersive combination with the active helix 1, to effectively tune the active helix to amplification of a relatively narrow band of frequencies only, and hence, prevent the generation of oscillations due to amplification of reflected waves of other frequencies outside the pass-band of the output circuit. This result is accomplished in Fig. 1 by making the external helix 23 dispersive. This can be done by periodically loading the helix 23 with uniformly-distributed reactances, in any of the ways disclosed in the abovementioned application of Dodds and Peter. By way of example, Fig. 1 shows the helix 23 loaded by means of conductors 25 connected between and shorting adjacent turns of the helix 23 at equal intervals therealong. The conductors 25 act as reactances attached to the helix 23, and produce periodically-recurring discontinuities in characteristic impedance in the wave path along the helix. Although the simple helix 1 would not by itself be dispersive, the close coupling to the dispersive external helix 23 causes the helix 1 to be dispersive, and hence, limits its amplifying bandwidth as desired. Since the external loaded helix 23 is closely coupled to the active inner helix, the axial wave or phase velocity of the two helices need not be the same.

Fig. 3 is a graph showing the insertion-loss or attenuation of a tube such as that of Fig. 1 as a function of frequency. The zero insertion loss line or base line represents substantially un-attenuated transmission of all frequencies in a conventional traveling wave tube with a non-dispersive helix. The use of a dispersive helix or a dispersive helix-filter combination introduces attenuation of those frequencies immediately above and below a band of operating frequencies having a center frequency f0, indicated by the vertical dotted line. The graph corresponds to: fo=3000 me; loss=7.5 db at in; gain=28 db; and a frequency range of 50 mc. The approximate var- 4 iation of helix wave velocity with frequency is shown by the slanting dotted line.

It has been found desirable to stretch the end turns of the loaded helix, to change the net wave velocity or change the type of loading at the ends, for matching purposes. Also, the sense of winding of the two helices may be chosen to be the same, or to be opposite, to control the magnitude of the dispersive eflfect. When a small amount of dispersion is sufiicient, a loaded helix having a wave velocity quite different from that of the active helix is satisfactory, provided the loaded helix has a pass band in the desired region.

Fig. 4 illustrates another embodiment of the invention wherein both the active helix 1 and external helix 27 are inherently non-dispersive, per se, but are so designed relative to each other that their combination is dispersive. In this case it is necessary that the axial phase velocity of the two helices be substantially the same. This requires that they have substantially the same ratio of circumference to pitch. Hence, the pitch of the largerdiameter outer helix must be greater than that of the smaller-diameter inner helix, as shown in the figure. In addition, the overall length of each of the helices must be substantially an integral number of half wavelengths at the center operating frequency, to make the combination of the two helices dispersive.

Fig. 5 shows an embodiment of the invention wherein the bandwidth limiting means is in the form of a branching filter composed of two transmission lines coupled to the helix at spaced points only. This filter employs the basic principles of a branching filter described by J. R. Pierce, Proc. 1. R. E., volume 37, pages 152-155, February, 1949, and schematically shown in Fig. 6, in which Z0 is the characteristic impedance of a primary circuit on which input signals are impressed, Z1 is the characteristic impedance of a secondary circuit, and 20 are impedances coupling the circuits together. The effect of the coupled secondary circuit is to extract a predetermined band of frequencies from the primary circuit.

Fig. 7 schematically shows the application of the principles of such a branching filter to a traveling wave tube, where Z0 is the characteristic impedance of the active helix of the tube, Z1 and Z2 are the characteristic impedances of two transmission lines forming parts of a branching filter, and Z61 and Z0 are the respective coupling impedances for the two lines. Fig. 8 is a graph showing the effect of the branching filter on the transmission characteristics of the active helix as a function of frequency. As shown, a relatively narrow band of frequencies near f0 are transmitted with little attenuation, and a relatively wide band of frequencies, centering at f1 and f2, on each side of f0 is extracted by the branching filter. Fig. 9 shows the approximate pass-band required of the helix output coupling, corresponding to the transmission or amplifying band of the active helix. Fig. 10 shows the approximate gain-frequency characteristic of the active helix in relation to the transmission characteristic of Fig. 8. It can be seen that frequencies below and above the rejection bands Afr and Afz in Fig. 8 will not be amplified sufiiciently to cause serious difiiculty due to reflections at the ends of the active helix.

Referring again to Fig. 5, a conventional helix 1 is supported within a glass envelope 3 and an electron beam is projected through the helix, as in Fig. l. A tubular shield 35 is disposed coaxially around and spaced from the envelope 3, the ends of the shield being connected to the input and output waveguides 5 and 17 In accordance with the invention, a plurality of coupling loops 37 are disposed around the envelope 3 at equally-spaced intervals along the major portion of the length of the helix 1. One end of each coupling loop 37 is connected to the shield 35, as shown. The other ends of the coupling loops 37 extend through apertures 39 in the shield 35 and form the inner conductors of coaxial transmission lines 41 and 43 which couple the loops 37 with two coaxial transmission 7 lines 45 and 47, respectively. Each of the lines 45 and 47 corresponds to the secondary circuit in the branching filter described by Pierce, and shown schematically in Fig. 6. Each of the four ends of the lines 45 and 47 is suitably terminated by lossy material 49, such as graphite, to absorb waves of those frequencies which are extracted from the primary circuit helix 1'.

The line 45 and the coupling lines 41 and loops 37 associated therewith are designed to extract from the helix one of the two undesired frequency bands, Afr or Afz in Fig. 8, on each side of the desired operating frequency range. Similarly, the other line 47 and the coupling lines 43 and loops 37 associated therewith are designed to extract the other undesired frequency band.

It will be understood that various modifications may be made in the tube of Fig. 5 without departing from the scope of the invention. More than two filter lines may be coupled to the helix to extract still other frequencies therefrom. Other types of transmission lines may be substituted for the coaxial lines used in the example shown.

Preferably, the signal is impressed upon and derived from the active inner helix 1, as in a conventional traveling wave tube. However, if desired, the signal input and output can be coupled directly to an external loaded helix 23, as shown in Fig. 11. In this case there are no axiallyextending coupling portions at the end of the active inner helix 53, so that the effective length of the active helix is increased. By way of example, the input and output means may be coaxial lines 55 and 57 coupled to the ends of the external loaded helix 23 and to the tubular shield 59.

The arrangement shown in Fig. 11 diifers from that disclosed in the above-mentioned copending application of R. W. Peter in that the external helix is inherently dispersive and also that the inner helix is the active helix with which the electron beam interacts.

Various combinations of the coupling arrangements shown in Figs. 1 and 11 can be employed. For example, the input signal may be coupled to either of the two helices, with the output signal derived from the other helix.

An important advantage inherent in all of the forms of the invention disclosed herein is that a standard tube design with a plain active helix can be inserted into any of several difierent filters to operate in different frequency ranges, analogous to the way in which a triode can be inserted into various tank circuits with different characteristic frequencies.

It will be apparent that the invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structures used and the purpose for which they are employed without departing from the scope of the invention as set forth in the following claims.

I claim:

1. An electron tube of the traveling wave type comprising an elongated conductive helix, means for projecting a beam of electrons through and in energy-coupling relation with the entire length of said helix, and means for limiting the amplifying bandwidth of said helix comprising at least one elongated transmission line inductively coupled to said helix at least at equally-spaced points distributed along substantially the entire length of said helix, and means causing the combination of said helix and said coupled transmission line to be dispersive along substantially the entire length thereof.

2. An electron tube according to claim 1, including input and output means coupled to the ends of only one of said helix and said transmission line.

3. An electron tube according to claim 2, wherein said signal input and output means are coupled to the ends of said helix only.

4. An electron tube according to claim 2 wherein said 6 input and output means are coupled to the ends of said transmission line only.-

5. An electron tube according to claim 1, wherein said transmission line is a second elongated conductive helix inductively coupled to said helix throughout substantially the entire length thereof.

6. An electron tube according to claim 1, wherein said transmission line is coupled to said helix by a plurality of branch lines and coupling loops equally spaced along said helix.

7. An electron tube of the traveling wave type comprising a first elongated conductive helix, means for projecting a beam' of electrons through and in energy-coupling rela: tion with the entire length of said first helix, a second elongated conductive helix inductively coupled to said first helix throughout substantially the entire length thereof, and means causing said second helix to be dispersive along substantially the entire length thereof to render the combination of said helices dispersive.

8. An electron tube of the traveling wave type comprising a first elongated conductive helix, means for projecting a beam of electrons along and in energy-coupling relation with the entire length of said first helix, a second elongated conductive helix inductively coupled to said first helix throughout substantially the entire length thereof, and a plurality of reactance elements distributed along substantially the entire length of said second helix at equally-spaced points thereon, to render the combination of said helices dispersive.

9. An electron tube according to claim 8, further including signal input and output means coupled to the ends of said second helix only.

10. An electron tube according to claim 8, wherein said reactance elements are conductors connected between and shorting adjacent turns of said second helix at said points.

11. An electron tube according to claim 1, wherein the ends of said transmission line are terminated by lossy material.

12. An electron tube of the traveling wave type adapted to operate at a predetermined frequency comprising a first elongated conductive helix of predetermined axial phase velocity, means for projecting a beam of electrons through the entire length of said first helix, and a second elongated conductive helix surrounding and closely coupled to said first helix along substantially the entire length thereof, said second helix having substantially the same axial phase velocity as said first helix, each of said helices having a length substantially equal to an integral number of half wavelengths at said operating frequency.

13. The combination of: an electron tube of the traveling wave type comprising an elongated dielectric envelope containing an elongated conductive helix, and means for projecting a beam of electrons through the entire length of said helix; and means for limiting the amplifying bandwidth of said tube comprising at least one elongated transmission line located outside said tube envelope and inductively coupled to said helix at least at equally-spaced points distributed along substantially the entire length of said helix, and means causing the combination of said helix and said coupled transmission line to be dispersive along substantially the entire length thereof.

14. The combination of: an electron tube of the traveling wave type comprising an elongated dielectric envelope containing an elongated conductive helix, and means for projecting a beam of electrons through the entire length of said helix; and means for limiting the amplifying bandwidth of said tube comprising at least one elongated transmission line located outside said tube envelope and inductively coupled to said helix at least at equally-spaced points distributed along substantially the entire length of said helix, and means causing said transmission line to be dispersive along substantially the entire length thereof.

15. The combination of: an electron tube of the travel ing wave type comprising an elongated dielectric envelope containing a first elongated conductive helix, and means for projecting a beam of electrons through the entire length of said helix; a second elongated conductive helix surrounding said tube envelope and inductively coupled to said first-mentioned helix along substantially the entire length thereof, and means causing said second helix to be dispersive along substantially the entire length thereof to render the combination of said helices dispersive.

16. The combination recited in claim 15, wherein said last-mentioned means comprises a plurality of reactance elements distributed along substantially the entire length of said second helix at equally-spaced points thereon.

17. The combination recited in claim 15, including signal input and output means coupled to the ends of said first helix only.

References Cited in the file of this patent UNITED STATES PATENTS Linder Mar. 23, Haefi" Feb. 25, Kleen et al. June 13, Linde'nblad Dec. 11, Tiley Feb. 5, Hansell Mar. 11, Hansell Mar. 11, 

